Nowadays, the wireless local area network (WLAN) is rapidly coming into widespread use, making the network environment in offices and homes wireless. For example, IEEE802.11a/g, which is a standard by Institute of Electrical and Electronics Engineers (IEEE), realizes a physical layer data rate of max 54 Mbps by using orthogonal frequency division multiplexing (OFDM) on frequencies in a 2.4 GHz band or 5 GHz band. In IEEE802.11n currently under development, high throughput (HT) exceeding 100 Mbps is about to be realized by further using space division multiplexing (SDM) through a multiple input multiple output (MIMO) channel.
MIMO is a communication method to realize a space multiplexed stream by providing a plurality of antenna elements on each of the transmitter and receiver sides (known). On a transmitting side, a plurality of pieces of transmission data is space/time-encoded and multiplexed and then distributed over a plurality of transmitting antennas before being transmitted to a channel. On a receiving side, by contrast, reception signals received by a plurality of receiving antennas via the channel are space/time-decoded and demultiplexed into the plurality of pieces of transmission data so that the original data can be obtained without crosstalk between streams. According to MIMO technology, for example, with an increasing number of streams to be spatially multiplexed by increasing the number of antennas of communication equipment, throughput per user can be improved while maintaining downward compatibility. However, further improvement of throughput for the whole of a plurality of users will be demanded in the future.
The IEEE802.11ac working group aims to develop a wireless LAN standard whose data transmission speed exceeds 1 Gbps by using a frequency band of 6 GHz or below and for realization thereof, a communication method like multi-user MIMO (MU-MIMO) and SDMA that shares wireless resources on the spatial axes among a plurality of users, that is, multiplexes frames addressed to a plurality of users at the same time in the directions of spatial axes is considered to be promising.
Currently, SDMA is under study as a basic technology of a next-generation mobile phone system based on time division multiple access (TDMA) such as PHS (Personal Handyphone System) and LTE (Long Term Evolution). While attention is being given, as described above, to one-to-many communication in the field of wireless LAN, there is almost no example of application thereof. This can be considered to be also based on the fact that it is difficult to efficiently multiplex a plurality of users in packet communication.
When SDMA is applied to the wireless LAN, a case in which variable length frames are multiplexed on the same time axis can be considered. This causes no problem when the transmission data length for each of the plurality of users has the same size for all, but if the frame length to be multiplexed varies due to differences of the transmission data length, the total transmission power abruptly changes accompanying an increase/decrease in the multiplexing number of frames in a period of transmission. If frames having different lengths are multiplexed and transmitted unchanged, problems may arise from various points of view, for example, reception power abruptly changes on the receiving side accompanying an increase/decrease in the multiplexing number of frames, inducing an unstable operation in terms of auto gain control (AGC), and power distribution within a frame regarding RCPI (Received Channel Power Indicator) standardized by IEEE802.11 becomes non-fixed. Thus, even if the original transmission data length for each user varies, frames multiplexed at the same time need to be transmitted in the end with the same frame length.
For example, in a system of the fixed frame format like a conventional cellular system, frames can be padded by insertion of data for diversity (see, for example, Patent Literature 1), scheduling of assigned time (see, for example, Patent Literature 2), variable data rate (see, for example, Patent Literatures 3 and 4), or variable channel configuration (see, for example, Patent Literature 5). On the contrary, since a system of the variable length frame format such as the wireless LAN has a basically different structure, it is difficult to apply these conventional technologies to such a system of the variable length frame format.
In a WLAN system, “burst” technology that continuously transmits a plurality of frames in the time direction is adopted for the purpose of improving frame efficiency. To implement the bursting, a space (Inter-Frame Space: IFS) is provided between consecutive frames. While a zero IFS (ZIFS) is used between consecutive frames using the same transmission power for immediate transmission, a reduced IFS (RIFS) is used when transmission power changes between frames. The RIFS is short when compared with other inter-frame spaces such as the short IFS (SIFS) and thus, a communicating station can continue to control channels. In IEEE802.11n, for example, the inter-frame space of 2 ms called RIFS is defined. In consideration of frame efficiency, the inter-frame space is preferably shorter.